docs: def download_mmcif_header(pdb_id, outdir='', force_rerun=False):
      """Download a mmCIF header file from the RCSB PDB by ID.
  
      Args:
          pdb_id: PDB ID
          outdir: Optional output directory, default is current working directory
          force_rerun: If the file should be downloaded again even if it exists
  
      Returns:
          str: Path to outfile
  
      """
      # TODO: keep an eye on https://github.com/biopython/biopython/pull/943 Biopython PR#493 for functionality of this
      # method in biopython. extra file types have not been added to biopython download yet
  
      pdb_id = pdb_id.lower()
      file_type = 'cif'
      folder = 'header'
      outfile = op.join(outdir, '{}.header.{}'.format(pdb_id, file_type))
  
      if ssbio.utils.force_rerun(flag=force_rerun, outfile=outfile):
          download_link = 'http://files.rcsb.org/{}/{}.{}'.format(folder, pdb_id, file_type)
          urlretrieve(download_link, outfile)
          log.debug('{}: saved header file'.format(outfile))
      else:
          log.debug('{}: header file already saved'.format(outfile))
  
      return outfile